Resonant tunneling diodes (RTDs) have the potential for use as compact and coherent terahertz (THz) sources operating at room temperature. In this paper, sub-THz and THz oscillators with RTDs integrated on planar circuits are described. Fundamental oscillation up to 0.65 THz and harmonic oscillation up to 1.02 THz were obtained at room temperature in our recent study. Limiting factors for oscillation frequency and output power are theoretically analyzed including tunneling and transit-time effects and parasitic elements. Oscillation frequency and its dependence on RTD size are in good agreement with the measured results. Based on this result, it is shown that fundamental oscillation up to 2.3 THz and an output power of 60 mW at 1 THz are theoretically expected by improving the structures of the RTD and the antenna. Voltage-controlled oscillation, which is useful for the precise control of frequency, is observed in the RTD oscillators. Coherent power combining in an array configuration to achieve high output power as well as mutual injection locking between the array elements are also described.

Fundamental oscillations up to 1.04 THz were achieved in resonant tunneling diodes at room temperature. A graded emitter and thin barriers were introduced in GaInAs/AlAs double-barrier resonant tunneling diodes for reductions of the transit time in the collector depletion region and the resonant tunneling time, respectively. Output powers were 7 μW at 1.04 THz and around 10 μW in 0.9–1 THz region. A change in oscillation frequency of about 4% with bias voltage was also obtained.

A large increase in oscillation frequency was achieved in resonant-tunneling-diode (RTD) terahertz oscillators by reducing the conduction loss. An n+-InGaAs layer under the air-bridge electrode connected to the RTD was observed to cause a large conduction loss for high-frequency current due to the skin effect. By introducing a new fabrication process removing the InGaAs layer, we obtained 1.92-THz oscillation, which extended the highest frequency of room-temperature electronic single oscillators. Theoretical calculations reasonably agreed with the experiment, and an oscillation above 2 THz is further expected with an improved structure of the slot antenna used as a resonator and radiator.

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Experimental and theoretical characteristics of sub-terahertz and terahertz oscillations in resonant tunneling diodes (RTDs) integrated with slot antennas are reported. In the experiments, oscillations up to 0.6 THz were obtained in GaInAs/AlAs double-barrier RTDs. The oscillation characteristics were theoretically analyzed for the total device structure including RTD and slot antenna. The equivalent circuit with all parasitic elements was taken into account for the RTD, and the actual structure of the antenna was analyzed using a three-dimensional electromagnetic simulator. The theoretical analysis was in good agreement with the measurements of oscillation frequency and output power. It was shown from the theoretical results that the RTD itself has the potential to oscillate up to 3.0 THz, and that the RTD oscillator with slot antenna is able to oscillate up to 2.8 THz if the device structure is optimized. The output power analysis showed that 90 µW at 1 THz is possible by optimizing the device structure.

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One THz harmonic oscillation was observed in a sub-THz oscillating GaInAs/ AlAs resonant tunneling diode integrated with a slot antenna. The fundamental and third-harmonic frequencies were 342 GHz and 1.02 THz, respectively, for a 50 m long antenna. The maximum output power of the fundamental mode was around 23 W, and that of the third-harmonic component was 2.6% of the fundamental. Theoretical analysis with the van der Pole equation qualitatively explained the measured results.